package main

import (
	"bufio"
	"bytes"
	"flag"
	"io"
	"os"
)

// StrongChars point to some strong chars for password
var StrongChars = []byte{'!', '_', '-', '&', '?'}

func isStrongChar(b byte) bool {
	for _, c := range StrongChars {
		if c == b {
			return true
		}
	}
	return false
}

func main() {
	n := flag.Int("n", 100, "byte number")
	flag.Parse()

	file, err := os.OpenFile("/dev/urandom", os.O_RDONLY, 644)
	if err != nil {
		panic(err)
	}

	buff := make([]byte, *n)
	reader := bufio.NewReader(file)
	for *n > 0 {
		b, err := reader.ReadByte()
		if err != nil {
			continue
		}
		if (b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9') || isStrongChar(b) {
			*n--
			buff[*n] = b
		}
	}
	buff[len(buff)-1] = '\n'
	io.Copy(os.Stdout, bytes.NewReader(buff))

}